altsvc: clone setting in curl_easy_duphandle

altsvc: clone setting in curl_easy_duphandle

The cache content is not duplicated, like other caches, but the setting
and specified file name are.

Test 1908 is extended to verify this somewhat. Since the duplicated
handle gets the same file name, the test unfortunately overwrites the
same file twice (with different contents) which makes it hard to check
automatically.

Closes #5923